Double bubble process for making strong, thin film

ABSTRACT

Polymeric, thin oriented films can be made by use of a combination of a hot blown process and a blown bubble process, i.e. a double bubble process. The resulting film has use in replacing PVC for in store stretch wrap applications. Very low density polyethylene, VLDPE, is an especially preferred material for this process. It may be used as a monolayer, or as one layer of a multilayer structure. In practicing the invention, the polymer is extruded and hot blown, and heated to a temperature above its orientation temperature. This heating can be accomplished by the use of a heated liquid reservoir or sock disposed at the lower end of the primary bubble, inside the bubble. The hot blown heated film is then passed through a first set of pinch rolls, re-inflated into a blown bubble, and collapsed at a second set of pinch rolls. Thin, tough and elastic packaging materials are obtained by this process.

This is a Divisional Application of application Ser. No. 161,007, filedDec. 2, 1993, now U.S. Pat. No. 5,456,929, which is a Divisional ofapplication Ser. No. 989,965, filed on Dec. 11, 1992, and issued as U.S.Pat. No. 5,298,202, which is a Continuation of application Ser. No.643,591, filed Jan. 18, 1991, now abandoned, which is aContinuation-In-Part of U.S. Ser. No. 494,699, filed Mar. 16, 1990, nowabandoned, which is a Continuation-In-Part of U.S. Ser. No. 387,056,filed Jul. 28, 1989, now abandoned.

BACKGROUND OF THE INVENTION

The present invention pertains to packaging film, and more particularlyto a thin polymeric film suitable for replacing polyvinyl chloride filmespecially in packaging applications.

Polyvinyl chloride (PVC) has long been used in many applications in thepackaging art. One particularly widespread application for PVC is theuse of such material as an overwrap material for trayed retail cuts ofmeat and other food products in a retail environment.

PVC has several desirable properties for this use. For example, it hasexcellent optics and good elasticity and stretch properties at usetemperatures.

Unfortunately, PVC also has several disadvantages, including theproduction of hydrogen chloride gas during heat sealing and thegenerally corrosive effects of such gases in the packaging room.

It would be of great benefit to the packaging industry, and particularto applications requiring an instore film for overwrapping trayed foodproducts, to provide a film with many of the advantages of PVC butwithout the disadvantages described above.

Applicant has discovered that a relatively thin polymeric film,preferably a polyolefin, and more preferably very low densitypolyethylene (VLDPE), produced by hot blowing and then stretch orientingthe extruded film, exhibits excellent elasticity, toughness, stretch andoptical properties.

The U.S. Pat. No. 3,456,044 (Pahlke) mentions thin films of thicknessesless than 1 mil such as 0.5 mils, and discloses a double bubble methodfor biaxially orienting thermoplastic films including the steps ofproducing a primary tubing which is inflated by introducing air into theinterior thereof, and a cooling ring 22, as well as squeeze rolls 34 and28, with rolls 34 having a greater speed than rolls 28. Between the twopairs of squeeze rolls is a reinflared secondary bubble. If annealing isdesired, the tubing can be reinflared to form a bubble 70.

U.S. Pat. No. 3,555,604 (Pahlke) is a patent based on a divisionalapplication which was derived from the same priority application as the'044 patent described above, and discloses the same informationdescribed above for the '044 patent.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method of making a thin,oriented polymeric film comprises extruding. the polymer as a meltstream through a tubular die; hot blowing the extruded film; heating thehot blown film to a temperature above its orientation temperature;directing the heated film through a first set of pinch rolls;reinflating the hot blown film by a blown bubble process; and collapsingthe reinflared film through a second set of pinch rolls.

In another aspect of the invention, a thin, oriented polymeric filmuseful as a packaging film is made by the steps of extruding the polymeras a melt stream through a tubular die; hot blowing the extruded film;heating the hot blown film to a temperature above its orientationtemperature; directing the heated film through a first set of pinchrolls; reinflating the hot blown film by a blown bubble process; andcollapsing the reinflated film through a second set of pinch rolls.

In still another aspect of the invention, a thin oriented packaging filmcomprises a layer of very low density polyethylene, the film having athickness of less than about 1 mil.

In yet another aspect of the invention, an apparatus for making thin,oriented polymeric film comprises means for extruding the polymer as amelt stream through a tubular die; means for hot blowing the extrudedfilm to form a primary bubble; means for heating the hot blown film to atemperature above its orientation temperature; a first set of pinchrolls through which the heated film is directed; a second set of pinchrolls by means of which the heated film, after reinflation by a blownbubble process, is collapsed; and means for circulating a heated liquidto and from a reservoir disposed at the lower end of the primary bubble.

Definitions

The term "polyolefin" is used herein in its stricter sense to mean athermoplastic polymer derived from simple olefins. Among these arepolyethylene, polypropylene and copolymers thereof with olefiniccomohomers. For example, very low density polyethylene may be considereda linear ethylene copolymer with a comonomer comprising such materialsas butene, hexene or octene. The term "polyolefin" is also used hereinin a broader sense to include copolymers of ethylene with comohomersthat are not themselves olefins, such as vinyl acetate (e.g. ethylenevinyl acetate copolymer or EVA).

The term "very low density polyethylene", or "VLDPE" is used herein todescribe linear ethylene alpha-olefin copolymer (flexomer) havingdensities of generally between about 0.880 and 0.915 grams/cubiccentimeter, and produced by a catalytic, low pressure process. "Ultralow density polyethylene" is also included in this term.

The term "ethylene vinyl acetate copolymer" (EVA) as used herein refersto a copolymer formed from ethylene and vinyl acetate monomers whereinthe ethylene derived units are present in major amounts and the vinylacetate derived units are present in minor amounts, generally one to 30percent by weight.

The terms "melt flow" and "melt index" are used herein to mean theamount, in grams, of a thermoplastic resin which is forced through anorifice of specified length and diameter in ten minutes under prescribedconditions in accordance with ASTM D 1238.

The term "flow rate ratio" (FRR) is used to mean a dimensionless numberderived by dividing the flow rate (melt flow or melt index) at oneCondition with the flow rate at another Condition (ASTM D 1238). FRR isindicative of molecular weight distribution. The higher the FRR, thebroader the molecular weight distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be further understood by reference to the drawingsherein, where:

FIG. 1 is a schematic diagram indicating the apparatus and process bywhich the film of the present invention is made; and

FIG. 2 is a graph showing the relationship between tensile force appliedto a film, and elongation, for different types of films including thedouble bubble film of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows schematically a device 10 for making the thin films of thepresent invention.

A melt stream of a polymeric material, preferably a material such asVLDPE is extruded through an extruder (not shown) and exits through adie 12 in a conventional manner. The extruded film is hot blown byconventional techniques to form a blown bubble 14.

Air cooling ring 16 positioned circumferentially around the blown bubbleat the position shown cools the thermoplastic melt as it exits die 12.

An optional auxiliary chilling ring 17 may also be positionedcircumferentially around the blown bubble down stream from air coolingring 16 to further chill the hot blown film.

The primary bubble 14 is melt oriented in both the machine andtransverse directions. Various blow up ratios may be used, butpreferably the primary bubble 14 is hot blown to a blow up ratio ofbetween 1.5 and 3.0.

The primary bubble 14 is collapsed at pinch rolls 16.

To assist in this process, guide plates 18 are positioned to theextremities of the blown bubble 14.

The collapsed bubble is then reinflared in a blown bubble process tostretch orient the blown and collapsed film. This is done in aconventional manner by trapping air or other hot gas within thesecondary bubble 20 so that the material stretches at its orientationtemperature transversely to impart further orientation of the materialin the transverse direction. The secondary bubble 20 is collapsed at asecond set of pinch rolls 22. A second set of guide plates 24 may beemployed to assist in the collapsing process.

The second set of pinch rolls 22 is rotated at a speed faster than thefirst set of pinch rolls 16 to impart stretch orientation in the machineor longitudinal direction to the thermoplastic material.

The recollapsed bubble 20 then passes from the second set of pinch rolls22 to take up roll 26.

The take up roll 26 may be a mill log which can be immediately stored orshipped to a distributor or customer, or may be stored for furtherprocessing such as slitting into single wound film, machine or naturalcenter fold film. Thus, as used herein the take up roll 26 representsany further processing, storage, or further modification of the doublewound, collapsed film once it exits the second set of pinch rolls 22,and is used herein to denote any of these possible further processingsteps.

When using very low density polyethylene, it was noticed that somethickened edges of the film resulted from this process. To correct thisdeficiency, it is preferred that a reservoir 28 of heated fluid bedisposed at the lower end of primary bubble 14 in such a way that thecollapsing material drawn through the primary set of pinch rolls 16 willpass in communication with the heated fluid. In this manner, the film ismore uniformly heated and temperature control can be achieved. Thickenedtape edges can be substantially avoided by such means.

Although the heated fluid of reservoir 28 is preferably hot water, othermedia may be used if temperatures above the 212° F. limit of hot waterare desired. For example propylene glycol (a food approved material),hot oil or hot emulsions may be used. One skilled in the art willunderstand that the exact nature of the heated fluid is not as criticalas its effectiveness in helping to uniformly heat the collapsing bubble14 as it is drawn through pinch rolls 16, and to uniformly heat thecollapsing bubble 14 to a temperature above its orientation temperature.

The heated fluid may also be an "active" substance which not onlyreheats the surrounding film, but also actually coats the interior ofthe bubble as it passes over the reservoir. An example is a hot wax orother functional coating.

The heated fluid may be recirculated from the reservoir through aheating means 30 by means of conduits 32 or other suitable transfermeans. Using the reservoir 28 of heated fluid, the types of materialswhich may be effectively used in the present process and device areincreased. These materials fall generally into three classes.

The first class consists of crystalline polymers which can be quenchedto the amorphous state by either air or an external cooling mandrel ofthe types depicted schematically at 16 and 17 of FIG. 1. Examples ofsuch materials are polyethylene terephthalate, saran, and polypropylene.In the case of polyethylene terephthalate and saran, hot water is asuitable liquid medium for use in reservoir 28. In the case ofpolypropylene, a higher temperature liquid medium is preferred.

A second class of materials suitable for the present process areamorphous polymers such as polyethylene terephthalate glycol (PETG),polystyrene, and polycarbonate. In the case of PETG, hot water is asuitable liquid medium for reservoir 28. Higher temperature liquid mediaare preferred for polystyrene and polycarbonate.

A third class of materials which can be used with the present processare crystalline polymers which are not quenched amorphous by using airor external mandrel means. An example is high density polyethylene. Inthis case, a temperature of at least about 240° F. for the medium ofreservoir 28 is required to melt out most of the crystalline structureof HDPE material.

Other materials, falling into one or more of these three classes, canalso be used in the present process. These include polyamide (nylon),ethylene vinyl alcohol copolymer, and polyvinyl chloride. Copolymers andblends of any of these materials can also be used.

A major advantage of this process and device of the present invention isthat a variety of polyolefinic materials may be made into relativelythin, oriented, tough materials suitable for packaging applications suchas in store tray overwrap. Film gauges of less than 1 mil, andpreferably less than 0.6 mils, even more preferably less than 0.5 milsand as low as 0.2 mils or less are obtainable. Even films withthicknesses as low as 0.1 mils or lower can be made by the practice ofthe present invention.

Although monolayer films are preferred, and particularly materials suchas very low density polyethylene, multilayer films may also be produced.This is both technically possible and also sometimes desirable dependingon the nature of the monolayer film. For example, a VLDPE available fromUnion Carbide and sold under the designation DFDA 1138 is a desirablematerial from a performance point of view. However, where materials havenot yet attained food contact status, outer layers of a food approvedmaterial such as EVA may be used to "sandwich" the VLDPE or other corematerial. When more than one layer is to be used, coextrusion techniquescommonly available in the art may be employed.

The invention may be further understood by reference to the followingexamples.

EXAMPLE 1

A central melt stream of very low density polyethylene (DFDA 1138available from Union Carbide) and outside melt streams of an EVA (Elvax3130 available from du Pont) were coextruded through a coextrusion dieand hot blown to a blow up ratio of 1.78. The hot blown film had athickness of 2 mils. The hot blown film was collapsed at a first set ofpinch rolls and then inflated between the first set of pinch rolls and asecond set of pinch rolls. The secondary bubble did not impart anysubstantial transverse orientation to the film material. The machinedirection orientation was increased by speeding up the second set ofpinch rolls relative to the first set of pinch rolls. The speed of thefirst set of pinch rolls was 36 feet per minute. The speed of the secondset of pinch rolls was 180 feet per minute. The result was a stablebubble with no transverse expansion and 5:1 machine direction expansionto produce a final film with a thickness of 0.4 mils.

EXAMPLE 2

A film was produced substantially like that of Example 1, with the sameconditions for the primary bubble (hot blown film). The secondary bubblewas inflated from 28 inch lay flat width to 46 inch lay flat width withtransverse expansion of 1.64:1.

The speed of the second set of pinch rolls was 108 feet per minute toeffect a machine direction expansion of the secondary bubble of 3:1. Thefilm collected had cold thick edges because of lack of temperaturecontrol, but the center had a gauge thickness of 0.20 mils.

EXAMPLE 3

The 0.20 mil fill of Example 2 was used to package cut beef in a foamtray. The fill exhibited strength and stretch characteristicsapproaching the characteristics of 60 mil stretch PVC. Hot wire cuttingwas similar to the PVC, but sealing required a heat-cool cycle toprevent the tabs from shrinking.

Packages made with the fill of Example 2 exhibited more elastic recoverythan PVC when poked with the finger.

It should be noted that when the heated hot blown fill is collapsedthrough nip roll 16 and expanded into secondary bubble 20, the fill canbe either monoaxially or biaxially stretched. Stretching in the machinedirection can be from ratios of 1:1 to 9:1. In the transverse direction,stretching ratios may range from 1:1 to 5:1. Any combination of rangeswithin those just described is possible. For example, the fill may beessentially unstretched in the machine direction (1:1) and biaxiallystretched at a 3:1 ratio in the transverse direction. The reverse isalso true.

Additionally, the degree of transverse direction or machine directionstretching in secondary bubble 20 is substantially independent of theblow ratio in primary bubble 14, so that many combinations of blow upratios in the primary and secondary bubble are achievable. However, morestress (higher bubble pressure) occurs in the secondary bubble if theblow ratio in the primary bubble is reduced. Therefore, higher blow upratios in the primary bubble will make it easiest to orient a givenmaterial in the secondary bubble.

Several additional fills were also produced by substantially the sameprocess as described in Examples 1 and 2.

A liquid reservoir or "sock" containing heated water, located at thelower interior portion of the hot blown primary bubble (see referencenumeral 28 of FIG. 1) was used to control the temperature of the hotblown film prior to re-inflation into the second bubble. Morespecifically, the heated water was maintained at temperatures between183° F. and 204° F. to raise the temperature of the hot blown film toabove its orientation temperature.

It has been found that the liquid reservoir not only brings thetemperature of the hot blown film up to its orientation temperature, butalso equilibrates the temperature of the film around the circumferenceof the bubble. This in turn provides for better control of the finalfilm gauge, and in particular reduces the incidence of thickened edgesin the final film.

In Table 2, a list of the film structures produced, and the blow upratios in the primary and secondary bubble are listed. The thickness ofthe final film, and temperature of the heated water inside the reservoirare also listed.

Table 1 describes materials utilized in Examples 3-17:

                  TABLE 1                                                         ______________________________________                                                COMMERCIAL                                                            RESIN   NAME        DESCRIPTION    COMPANY                                    ______________________________________                                        EVA.sub.1                                                                             ELVAX 3130  EVAl2% VA      DU PONT                                                        MELT INDEX =                                                                  2.5 grams/                                                                    10 min.                                                   EVA.sub.2                                                                             32.89       EVA4% VA       EXXON                                                          MELT INDEX =                                                                  10 grams/                                                                     10 min.                                                   EVA.sub.3                                                                             9653        EVA12% VA      DU PONT                                                        (including                                                                    antifog                                                                       additive)                                                 VLDPE.sub.1                                                                           DFDA 1138   .900 grams/cc  UNION                                                          MELT INDEX =   CARBIDE                                                        0.4 grams/                                                                    10 min.                                                   VLDPE.sub.2                                                                           1121        DENSITY =      UNION                                                          .900 grams/cc  CARBIDE                                                        MELT INDEX =                                                                  0.4 grams/                                                                    10 min.                                                                       (ANTIFOG VERSION                                                              OF DFDA 1138)                                             VLDPE.sub.3                                                                           4003        DENSITY =      DOW                                                            .905 grams/cc                                                                 MELT INDEX =                                                                  .8 grams/10 min.                                          VLDPE.sub.4                                                                           XUR 1567    DENSITY =      DOW                                                41225-18K   .900 grams/cc                                                                 MELT INDEX =                                                                  .8 grams/10 min.                                          VLDPE.sub.5                                                                           DEFD 1491   DENSITY =      UNION                                                          .900 grams/cc  CARBIDE                                                        MELT INDEX =                                                                  grams/10 min.                                             ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________                                       SECONDARY                                                                            THICKNESS                                                                            SOCK                         EXAMPLE                                                                             FILM STRUCTURE         PRIMARY                                                                             T   L  (MILS) TEMP(°F.)             __________________________________________________________________________    4     EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1                                                                    1.7:1 3:1 3.8:1                                                                            .25    183                          5     EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1 (1/2 RATE).sup.1                                                   1.7:1 3:1 3.8:1                                                                            .12    183                          6     EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1                                                                    2.5:1 2.1:1                                                                             3.9:1                                                                            .30    184                          7     EVA.sub.2 /VLDPE.sub.1 /EVA.sub.2                                                                    2.5:1 2.1:1                                                                             3.9:1                                                                            .30    204                                (LOWER SKIN RATES)                                                      8     50% EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 +                                                          2.5:1 2.1:1                                                                             3.9:1                                                                            .30    184                                +50% EVA.sub.1 //50% EVA.sub.1                                          9     50% EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 +                                                          2.0:1 2.6:1                                                                             3.4:1                                                                            .30    184                                +50% EVA.sub.1 //50% EVA.sub.1                                          10    50% EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 +                                                          1.9:1 2.8:1                                                                             3.0:1                                                                            .30    190                                +50% EVA.sub.1 //50% EVA.sub.1                                          11    50% EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 + (LOWER                                                   1.9:1.sup.2                                                                         2.8:1                                                                             3.0:1                                                                            .18    190                                +50% EVA.sub.1 //50% EVA.sub.1                                          12    50% EVA.sub.3 /VLDPE.sub.2 /VLDPE.sub.1 /VLDPE.sub.2 /50% EVA.sub.3           +                      1.9:1 2.8:1                                                                             3.0:1                                                                            .30    195                                +50% EVA.sub.1 //50% EVA.sub.1                                          13    50% EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 +                                                          1.9:1 3.0:1                                                                             3.0:1                                                                            .30    195                                +50% EVA.sub.1 //50% EVA.sub.1                                          14    EVA.sub.3 /VLDPE.sub.1 /50% EVA.sub.3 + (FASTER DRAW).sup.3                                          1.9:1 3.0:1                                                                             3.1:1                                                                            .18    195                                +50% EVA.sub.1 //50% EVA.sub.1                                          15    EVA.sub.1 /VLDPE.sub.3 /EVA.sub.1                                                                    2.1:1 2.7:1                                                                             3.0:1                                                                            .25    195                          16    50% EVA.sub.3 /VLDPE.sub.4 /50% EVA.sub.3 +                                                          1.9:1 2.7:1                                                                             2.8:1                                                                            .30    195                                +50% EVA.sub.1 //50% EVA.sub.1                                          17    EVA.sub.1 /VLDPE.sub.5 /EVA.sub.1                                                                    1.9:1 3.0:1                                                                             3.0:1                                                                            .25    195                          __________________________________________________________________________     .sup.1 All extruders were reduced to one half the rate of the extruder of     Example 4, i.e. 41 rpm of Example 5.                                          .sup.2 All extruders were reduced to 60% of the rate of the extruders of      Example 4, i.e. 50 rpm for Example 11.                                        .sup.3 The takeoff speed of the film was 59% faster than that of Example      4, i.e. 200 feet/min. for Example 14.                                    

In Table 4, a list of other film structures produced, and the blow upratios in the primary and secondary bubble are listed. The thickness ofthe final film, and temperature of the heated water inside the reservoirare also listed.

Table 3 describes materials utilized in Examples 18-50. Other materialsused in Examples 18-50 are listed and described in Table 1.

                  TABLE 3                                                         ______________________________________                                                COMMERCIAL                                                            RESIN   NAME        DESCRIPTION    COMPANY                                    ______________________________________                                        EVA.sub.4                                                                             ELVAX 3165  EVA18% VA      DU PONT                                                        MELT INDEX =                                                                  .7 grams/                                                                     10 min.                                                   EVA.sub.5                                                                             ELVAX 3170  EVA18% VA      DUPONT                                                         MELT INDEX =                                                                  2.5 grams/                                                                    10 min.                                                   EVA.sub.6                                                                             ELVAX 3182  EVA28% VA      DU PONT                                                        MELT INDEX =                                                                  3.0 grams/                                                                    10 min.                                                   EVA.sub.7                                                                             ELVAX 3508  EVA12% VA      DUPONT                                                         MELT INDEX =                                                                  .3 grams/                                                                     10 min.                                                   EAA.sub.1                                                                             PRIMACORE   EAA9.5% AA     DOW                                                1410                                                                  VLDPE.sub.6                                                                           FW 1990     DENSITY =      NORSOLOR                                                       .900 grams/cc                                                                 MELT INDEX =                                                                  .8 grams/                                                                     10 min.                                                   VLDPE.sub.7                                                                           30S         DENSITY = .901 DOW                                                            grams/cc                                                                      MELT INDEX =                                                                  .8 GRAMS/                                                                     10 min.                                                   VLDPE.sub.8                                                                           30R         DENSITY = .902 DOW                                                            grams/cc                                                                      MELT INDEX =                                                                  .6 grams/                                                                     10 min.                                                   VLDPE.sub.9                                                                           30Q         DENSITY = .895 DOW                                                            grams/cc                                                                      MELT INDEX =                                                                  .42 grams/                                                                    10 min.                                                   ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________                                  SECONDARY                                                                            THICKNESS                                                                            SOCK                              EXAMPLE                                                                             FILM STRUCTURE    PRIMARY                                                                             T   L  (MILS) TEMP(°F.)                  __________________________________________________________________________    18    50% EVA.sub.3 /VLDPE.sub.5 /50% EVA.sub.3.sup.4                                                 2.1:1 2.7:1                                                                             3:1                                                                              .3     195                                     +50% EVA.sub.2 //+50% EVA.sub.1                                         19    50% EVA.sub.3 /VLDPE.sub.5 /50% EVA.sub.3.sup.5                                                 2.1:1 2.7:1                                                                             3:1                                                                              .3     195                                     +50% EVA.sub.4 //+50% EVA.sub.4                                         20    25% EVA.sub.3 /VLDPE.sub.5 /25% EVA.sub.3.sup.6                                                 2.1:1 2.7:1                                                                             3:1                                                                              .3     195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         21    50% EVA.sub.3 /VLDPE.sub.5 /50% EVA.sub.3                                                       2.1:1 2.7:1                                                                             3:1                                                                              .3     195                                     +50% VLDPE.sub.5 //+50% VLDPE.sub.5.sup.7                               22    EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1                                                               2.1:1 2.7:1                                                                             2.3:1                                                                            .25    195                               23    EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1.sup.8                                                         2.9:1 0   3:1       195                               24    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.9                                                 2.5:1 2.3:1                                                                             2:1                                                                              .50    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         25    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.10                                                2.5:1 2.3:1                                                                             2:1                                                                              .50    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         26    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.11                                                1.9:1 3:1 2:1                                                                              .4     195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         27    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.12                                                1.9:1 3:1 2:1                                                                              .4     195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         28    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.13                                                1.9:1 3:1 2:1                                                                              .4     195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                                                 2.2:1                                                 29    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3                                                       2.2:1 3:1 2:1                                                                              .25    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         30    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.14                                                1.9:1 3:1 2:1                                                                              .5     195                                     +75% EVA.sub.5 //+75% EVA.sub.5                                         31    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.15                                                1.9:1 3:1 2:1                                                                              .25    195                                     +75% EVA.sub.5 //+75% EVA.sub.5                                         32    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.16                                                1.9:1 3:1 2:1                                                                              .50    181                                     +75% EVA.sub.6 //+75% EVA.sub.6                                         33    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.17                                                1.9:1 3:1 2:1                                                                              .5     195                                     +75% EAA.sub.1 //+75% EAA.sub.1                                         34    EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1.sup.18                                                        1.9:1 3:1 2:1                                                                              1.0    195                               35    EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1.sup.19                                                        2.2:1 1.5:1                                                                             1:1       195                               36    EVA.sub.1 /VLDPE.sub.1 /EVA.sub.1.sup.20                                                        2.1:1 2.1:1                                                                             1:1       195                               37    EVA.sub.7 /VLDPE.sub.5 /EVA.sub.7.sup.21                                                        3:1                                                   38    EVA.sub.1 /VLDPE.sub.6 /EVA.sub.1.sup.22                                                        2.4:1 2.3:1                                                                             2:1                                                                              .5     193                               39    EVA.sub.1 /VLDPE.sub.6 /EVA.sub.1.sup.23                                                        2.4:1 2.3:1                                                                             2:1                                                                              .25    193                               40    EVA.sub.1 /VLDPE.sub.7 /EVA.sub.1                                                               2.1:1 2.7:1                                                                             2:1                                                                              .5     195                               41    EVA.sub.1 /VLDPE.sub.7 /EVA.sub.1.sup.24                                                        2.1:1 2.7:1                                                                             2:1                                                                              .25    195                               42    25% EVA.sub.3 /VLDPE.sub.8 /25% EVA.sub.3.sup.25                                                2.0:1 2.8:1                                                                             2:1                                                                              .25    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         43    25% EVA.sub.3 /VLDPE.sub.8 /25% EVA.sub.3.sup.26                                                2.9:1 1.9:1                                                                             2:1                                                                              .25    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         44    25% EVA.sub.3 /VLDPE.sub.8 /25% EVA.sub.3.sup.27                                                2.0:1 2.8:1                                                                             2:1                                                                              .25    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         45    VLDPE.sub.1.sup.28                                                                              2.0:1 2.7:1                                                                             2:1                                                                              .50    195                               46    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.29                                                1.9:1 3:1 2:1                                                                              .25    195                                     +75% EVA.sub.7 //+75% EVA.sub.7                                         47    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.30                                                1.9:1 2.9:1                                                                             2:1                                                                              .25    195                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         48    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.-                                                 1.9:1 2.9:1                                                                             3:1                                                                              .25    193                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         49    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.32                                                2.3:1 2.5:1                                                                             2.5:1                                                                            .25    193                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         50    25% EVA.sub.3 /VLDPE.sub.1 /25% EVA.sub.3.sup.33                                                1.9:1 2.9:1                                                                             3:1                                                                              .25    193                                     +75% EVA.sub.1 //+75% EVA.sub.1                                         __________________________________________________________________________     .sup.4 Die slough problem (new air ring) die gap. 110".                       .sup.5 Reduced die slough. Couldn't maintain bubble.                          .sup.6 No die slough, bubble life 15 min.                                     .sup.7 Mottled coex. Secondary bubble life short.                             .sup.8 L. O. film.                                                            .sup.9 Reduced die gap from .110 to .075 2 hr. bubble. Couldn't make .25      mil.                                                                          .sup.10 Changed die gap to .040". Primary bubble temp. from die to sock       300, 173, 160, 186  Still unstable orientation. Changed to Johnson air        ring.                                                                         .sup.11 Die gap = .075". Primary bubble temp. from die to sock 279, 188,      138, 191. Secondary bubble gauge = .2-.6                                      .sup.12 Adjusted Gauge: Secondary: .35-.5                                     Primary: .16-.27                                                              Primary bubble gauge offbalanced to achieve better gauge.                     Probably due to skin layer thickness variations orienting differently.        .sup.13 Turned off chiller causing primary bubble to widen.                   .sup.14 Chilled air  Johnson air ring.                                        .sup.15 Some incompatibility haze but ran well fewer gels causing bubble      breaks.                                                                       .sup.16 Serious incompatability.                                              .sup.17 Hard to process.                                                      .sup.18 Made for low cost forming web.                                        .sup.19 Transverse orientation.                                               .sup.20 Transverse maximum (obstruction limited).                             .sup.21 Could not maintain bubble because of apparent excess primary          orientation, hot blown instead.                                               .sup.22 Amorphous clear primary bubble.                                       .sup.23 Primary bubble pulsed. Switched core to (LW2550). Primary melt wa     too weak to make film.                                                        .sup.24 Primary bubble too weak.                                              .sup.25 Stable bubble.                                                        .sup.26 More stable than 41.                                                  .sup.27 Very stable (gels might have been due to EVA)                         .sup.28 Stable orientation, hazy film.                                        .sup.29 Progressed to hazy weak film. Melt became cheesy. Suspected           Xlinking in die or extruder.                                                  .sup.30.1% Irganox 1010 added to skins.                                       .sup.31 Increased L. O.                                                       .sup.32 Lower transverse & L. O. ratio.                                       .sup.33 Increased die from 350 to 375° F. Conclusion: Irganox 1010     reduced Xlinking of skin layers.                                         

In Table 6, a list of additional film structures produced, and a blow upratio in the primary and secondary bubble are listed. The thickness ofthe final film, and temperature of the heated water inside the reservoirare also listed.

Table 5 describes materials utilized in Examples 51 through 76. Othermaterials used in Examples 51 through 76 are listed and described inTables 1 and 3.

                  TABLE 5                                                         ______________________________________                                               COMMERCIAL                                                             RESIN  NAME        DESCRIPTION     COMPANY                                    ______________________________________                                        VLDPE.sub.10                                                                         TAFMER 0680 DENSITY = .88   MITSUI                                                        grams/cc                                                                      MELT INDEX =                                                                  .4 grams/                                                                     10 min.                                                    VLDPE.sub.11                                                                         DEFD 1064 NT 7                                                                            DENSITY = .912  UNION                                                         grams/cc        CARBIDE                                                       MELT INDEX =                                                                  1.0 grams/                                                                    10 min.                                                    VLDPE.sub.12                                                                         1210        DENSITY = .89   UNION                                                         grams/cc        CARBIDE                                                       Narrow Molecular                                                              Weight Distribution                                                           MELT INDEX =                                                                  1 gram/                                                                       10 min.                                                    VLDPE.sub.13                                                                         61509.32    DENSITY = .911  DOW                                                           grams/cc                                                                      MELT INDEX =                                                                  .55 grams/                                                                    10 min.                                                    SBC.sub.1                                                                            KR-10       STYRENE BUTADIENE                                                                             PHILLIPS                                                      COPOLYMER                                                                     DENSITY = 1.01                                                                grams/cc                                                                      MELT INDEX =                                                                  8 grams/                                                                      10 min.                                                    ______________________________________                                    

                                      TABLE 6                                     __________________________________________________________________________                                   SECONDARY                                                                             THICKNESS                                                                            SOCK                            EXAMPLE                                                                             FILM STRUCTURE     PRIMARY                                                                             T   L   (MILS) TEMP(°F.)                __________________________________________________________________________    51    50% EVA.sub.1 /VLDPE.sub.5 /50% EVA.sub.1.sup.34                              +50% EVA.sub.3 //+50% EVA.sub.3                                         52    50% EVA.sub.1 /EVA.sub.7 /50% EVA.sub.1.sup.35                                +50% EVA.sub.3 //+50% EVA.sub.3                                         53    50% EVA.sub.1 /VLDPE.sub.3 /50% EVA.sub.1.sup.36                              50% EVA.sub.3 //+50% EVA.sub.3                                          54    50% EVA.sub.7 /VLDPE.sub.3 /50% EVA.sub.7.sup.37                              +50% EVA.sub.3 //+50% EVA.sub.3                                         55    50% EVA.sub.7 /80% VLDPE.sub.5 /50% EVA.sub.7.sup.38                                             2.1:1 2.70:1                                                                            2.85:1     195                                   +50% EVA.sub.3 /+20% VLDPE.sub.3 /+50% EVA.sub.3                        56    50% EVA.sub.1 /80% VLDPE.sub.5 /50% EVA.sub.1.sup.39                                             2.1:1 2.70:1                                                                            2.85:1     195                                   +50% EVA.sub.3 /+20% VLDPE.sub.3 /+50% EVA.sub.3                        57    50% EVA.sub.7 /80% VLDPE.sub.10 /50% EVA.sub.7.sup.40                                            2.1:1 2.75:1                                                                            2.83:1     195                                   +50% EVA.sub.3 /+20% VLDPE.sub.3 /+50% EVA.sub.3                        58    50% EVA.sub.7 /65% VLDPE.sub.30 /50% EVA.sub.7.sup.41                                            2.10:1                                                                              2.7:1                                                                             2.85:1     195                                   +50% EVA.sub.1 /35% VLDPE.sub.12 /+50% EVA.sub.3                        59    50% EVA.sub.3 /65% VLDPE.sub.11 /50% EVA.sub.1.sup.42                                            2.23:1                                                                              2.67:1                                                                            2.88:1                                                                            .20    211                                   +50% EVA.sub.3 /35% VLDPE.sub.12 /+50% EVA.sub.3                        60    50% EVA.sub.1 /65% VLDPE.sub.11 /50% EVA.sub.1.sup.43                                            2.55:1                                                                              1.00:1                                                                            3.00:1                                                                            .70    211                                   +50% EVA.sub.3 /35% VLDPE.sub.12 /+50% EVA.sub.3                        61    50% EVA.sub.1 /VLDPE.sub.11 /50% EVA.sub.1.sup.44                                                2.23:1                                                                              2.67:1                                                                            2.88:1     211                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         62    VLDPE.sub.11 /VLDPE.sub.11 /VLDPE.sub.11.sup.45                                                  2.33:1                                                                              1.00:1                                                                            3.00:1                                                                            1.50   200                             63    VLDPE.sub.11 /VLDPE.sub.11 /VPDPE.sub.11                                                         3.29:1                                                                              1.90:1                                                                            3.43:1     210                             64    50% EVA.sub.1 /VLDPE.sub.11 /50% EVA.sub.1                                                       3.29:1                                                                              1.90:1                                                                            3.00:1     209                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         65    50% EVA.sub.1 /VLDPE.sub.11 /50% EVA.sub.1.sup.46                                                2.23:1                                                                              2.67:1                                                                            2.88:1     211                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         66    50% EVA.sub.1 /VLDPE.sub.11 /50% EVA.sub.1                                                       2.23:1                                                                              2.67:1                                                                            3.00:1                                                                            .20    211                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         67    50% EVA.sub.1 /65% VLDPE.sub.11 /50% EVA.sub.1                                                   2.33:1                                                                              2.5:1                                                                             2.65:1                                                                            .40    207                                   +50% EVA.sub.3 /+35% EVA.sub.4 /+50% EVA.sub.3                          68    50% EVA.sub.1 /VLDPE.sub.13 /50% EVA.sub.1                                                       2.12:1                                                                              2.90:1                                                                            3.00:1                                                                            .25    209                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         69    50% EVA.sub.1 /VLDPE.sub.13 /50% EVA.sub.1                                                       1.60:1                                                                              3.73:1                                                                            3.00:1                                                                            .20    209                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         70    50% EVA.sub.1 /VLDPE.sub.1 /50% EVA.sub.1                                                        2.12:1                                                                              3.00:1                                                                            3.00:1                                                                            .20    205                                   +50% EVA.sub.3 //+50% EVA.sub.3                                         71    EVA.sub.5 /VLDPE.sub.1 /EVA.sub.5.sup.47                                                         2.12:1                                                                              3.00:1                                                                            3.00:1                                                                            .20    205                             72    EVA.sub.4 /VLDPE.sub.1 /EVA.sub.4.sup.48                                                         2.02:1                                                                              3.05:1                                                                            3.00:1                                                                            .20                                    73    EVA.sub.4 /80% VLDPE.sub.13 /EVA.sub.4.sup.49                                                    2.02:1                                                                              2.90:1                                                                            3.00:1                                                                            .20    211                                   +20% VLDPE.sub.10                                                       74    50% EVA.sub.1 /80% VLDPE.sub.13 /50% EVA.sub.1.sup.50                                            2.02:1                                                                              2.90:1                                                                            3.00:1                                                                            .20    211                                   +50% EVA.sub.3 /+20% VLDPE.sub.10 /+50% EVA.sub.3                       75    EVA.sub.4 /80% VLDPE.sub.13 /EVA.sub.4.sup.51 /                                                  2.02:1                                                                              2.90:1                                                                            3.00:1                                                                            .20    211                                   +20% EVA.sub.4                                                          76    SBC.sub.1 /EVA.sub.6 /80% VLDPE.sub.13 /EVA.sub.6 /SBC.sub.1.sup.52           /                  2.97:1                                                                              1.00:1                                                                            2.50:1                                                                            1.00   210                                   +20% EVA.sub.4                                                          __________________________________________________________________________     Notes For Table 6                                                             .sup.34 Primary bubble too weak.                                              .sup.35 Better primary bubble strength but secondary was unstable.            .sup.36 Could not maintain secondary bubble.                                  .sup.37 Better primary stability but secondary still unstable.                .sup.38 Slightly hazy film with 20 min. bubble life.                          .sup.39 Primary lost some stability secondary bubble life 20 min. better      clarity.                                                                      .sup.40 Up to 2 hr. bubble (blend approach promising).                        .sup.41 2000 ft. run when pumps failed.                                       .sup.42 Hazy film, hard to blow secondary but stable when blown.              .sup.43 L. O. material.                                                       .sup.44 Easier to blow than Sample 5. Stable secondary but hazy film.         .sup.45 L. O. material.                                                       .sup.46 Stable but hazy bubble.                                               .sup.47 Lost feed on skin layers after a short time.                          .sup.48 18% VA skin layers contributed to best stability yet.                 .sup.49 Best and most stable process yet. No cold stretch line as in 13 &     14. Very tough primary.                                                       .sup.50 Could only maintain 10 min. bubbles plus primary was not as tough     .sup.51 Became hazy due to EVA blend.                                         .sup.52 Clear monax for label trials.                                    

The inventor has found that materials with high molecular weight andwide molecular weight distribution are preferred for the core layer ofmultilayer fill structures in accordance with the invention. Thesematerials, particularly VLDPE, are also characterized by lowcrystallinity at processing temperatures.

Preferred VLDPE resins are characterized by high molecular weight (i.e.relatively low melt index), broad molecular weight distribution (i.e.relatively high flow rate ratio), and relatively low crystallinity atprocessing temperatures.

For the VLDPE, a melt index (MI) of no more than about 0.15 grams/10minutes (ASTM D 1238) (Condition 190/2.16) is preferred. A morepreferred MI is 0.12 grams/10 minutes.

Preferred VLDPE resins can also be characterized by a melt index of nomore than about 0.50 grams/10 minutes, more preferably no more thanabout 0.45 grams/10 minutes (ASTM D 1238) (Condition 190/5.0); no morethan about 1.50 grams/10 minutes, and more preferably no more than about1.35 grams/10 minutes (ASTM D 1238) (Condition 190/10.); or no more thanabout 10 grams/10 minutes, and more preferably no more than about 6grams/10 minutes (ASTM D 1238) (Condition 190/21.601).

For the VLDPE, a molecular weight distribution (flow rate ratio) of atleast about 10 (I₂₁ /I₅) (ASTM D 1238) is preferred. This value isderived by dividing the flow rate at Condition 190/21.6 by the flow rateat Condition 190/5.0. A more preferred FRR is 13.

Preferred VLDPE resins can also be characterized by a FFR of at leastabout 40, more preferably at least about 50 (I₂₁ /I₂.1) (ASTM D 1238).This value is derived by dividing the flow rate at Condition 190/21.6 bythe flow rate at Condition 190/2.16.

An especially preferred VLDPE is a high molecular weight resin such asDEFD 1015 from Union Carbide. This resin has a density of about 0.900grams/cc, a melt flow rate of about 6 grams/10 minutes (ASTM D 1238,Condition 190/21.601), and a FFR (I₂₁ /I_(2/1)) of about 50.

It has also been found that high vinyl acetate EVA resins i.e. 18% andhigher, are preferred for use in the outer layers of multilayer films ofthe present invention. These higher vinyl acetate EVA resins provide awider sealing range for the final film when used in in-store overwrapapplications on conventional manual and automatic equipment.

Although the invention has been described with respect to specificembodiments, those skilled in the art will understand that suitablemodifications and changes may be made without departing from the spiritand scope of the invention as defined below.

For example, the oriented film produced by the process of the presentinvention has a relatively high free shrink and shrink force. At 185°F., a typical free shrink value for the film of the present invention is50%. In end-use applications where it is desirable to reduce the amountof free shrink or shrink force or shrink tension in the film, thematerial exiting the secondary bubble may be annealed or heat set bymeans of a tertiary bubble. This bubble may be characterized as one ofintermediate pressure, compared with the relatively high pressuresecondary bubble, and the relatively low pressure primary bubble.

Films produced by the present double bubble process can be graphicallycharacterized as shown in FIG. 2. Because of the nature of the process,a thin film is produced that has excellent "memory" combined with goodelongation. This combination of properties compares favorably withconventionally oriented film which has good memory but poor elongationproperties. The present film also compares favorably with conventionalhot blown films which exhibit good elongation but poor memory.

Films of the present invention can optionally be cross-linked. This canbe done chemically or by the use of irradiation.

Irradiation may be accomplished by the use of high energy electrons,ultra violet radiation, X-rays, gamma rays, beta particles, etc.Preferably, electrons are employed up to about 20 megarads (MR) dosagelevel. The irradiation source can be any electron beam generatoroperating in a range of about 150 kilovolts to about 6 megavolts with apower output capable of supplying the desired dosage. The voltage can beadjusted to appropriate levels which may be for example 1,000,000 or2,000,000 or 3,000,000 or 6,000,000 or higher or lower. Many apparatusfor irradiating films are known to those of skill in the art. Theirradiation is usually carried out at a dosage up to about 20 MR,typically between about 1 MR and about 20 MR, with a preferred dosagerange of about 2 MR to about 12 MR. Irradiation can be carried outconveniently at room temperature, although higher and lowertemperatures, for example, 0° C. to 60° C. may be employed.

What is claimed is:
 1. A biaxially oriented packaging film having athickness of less than about 0.10 mils comprising a layer of a polymericmaterial selected from the group consisting of:a) polyethyleneterephthalate; b) polyethylene terephthalate glycol; c) polycarbonate;and d) mixtures of any of these materials.
 2. The film of claim 1wherein the film is crosslinked.